Multi-coil-on-plug secondary ignition (negative-fired)

How to perform the test

Some vehicles are equipped with a Distributorless Ignition System where the coils are combined into a coil-on-plug unit that sits directly on top of the spark plugs and houses the coil ignition system. This was commonly fitted to SAAB engines as well as certain Vauxhalls, Peugeots and others, as shown in Figure 1.

Disconnect the multiplug from the coil-on-plug unit and remove the unit from the engine.

Using the PP339 HT Extension Leads*, connect all the coil outputs to the spark plugs as shown in Figures 2 and 3.

Once all the HT Extension Leads have been fitted, connect the multiplug back onto the coil-on-plug unit using suitable breakout leads as shown in Figures 2 and 3.

An earth cable is supplied with the PP339 HT Extension Lead set. Connect one end using the supplied nut and bolt to the mounting hole of the COP unit and attach the other end to a suitable earth point, such as the COP unit mounting point on the engine, as shown in Figure 2. This will ensure that if the coil has a ground return through the coil pack then this is maintained. It will also safely conduct any exposed sparks to ground, to protect both the user and the equipment.

Connect a Secondary Ignition Pickup to Channel A of the PicoScope.

Connect the secondary pickup clip to the HT Extension lead and connect the ground clip to a suitable ground point on the engine or chassis as shown in Figures 2 and 3.

With the engine running, an ignition pattern similar to the example below should appear on the screen.

If a waveform cannot be seen, this could be because the output is positive-fired. If the HT pickup is moved to another HT extension lead, the waveform should then appear.

On a 4-cylinder engine with this type of ignition system it is common to have two negative-fired outputs and two positive-fired.

*Note: the Advanced Kit has 4 of these leads included as standard. Extra leads can be purchased for working on vehicles such as SAABs with more than 4 cylinders, but you can only test up to 4 at one time if using a 4 Channel Automotive Scope.

Example waveform

Waveform notes

The example waveform shows a typical picture from an engine with electronic ignition. The waveform is from the coil-on-plug unit on the Vectra Z22SE Engine.

The secondary waveform shows the length of time for which the HT flows across the spark plug's electrode after the initial peak of voltage required to jump the plug gap. This time is referred to as either the 'burn time' or the 'spark duration'. In the illustration, the horizontal voltage line in the centre of the oscilloscope is at fairly constant voltage, but then drops sharply into what is referred to as the 'coil oscillation' period. The 'burn time' is also illustrated in Figure 4.

The coil oscillation period (as illustrated in Figure 5) should display at least 4 peaks (including upper and lower). A loss of peaks indicates that the coil needs substituting. The period between the coil oscillation and the next 'drop down' is when the coil is at rest and there is no voltage in the coil's secondary circuit. The 'drop down' is referred to as the 'negative polarity peak', (as illustrated in Figure 6) and produces a small oscillation in the opposite direction to the plug firing voltage. This is due to the initial switching on of the coil's primary current. The voltage within the coil is only released at the correct point of ignition when the HT spark ignites the air/fuel mixture.

The plug firing voltage is the voltage required to jump the gap at the plug's electrode, commonly known as the 'plug kV'. This is shown in Figure 7. In this example the plug kV is 13.5 kV.

Figure 8 - Example multi-coil-on-plug unit

Technical information

The operation of the coil-on-plug Unit is essentially the same as any other ignition system.

The distributorless ignition systems are fitted only to vehicles that have an even number of cylinders such as 2, 4, 6 or 8. This is because two cylinders are connected to one coil that can produce a spark to both cylinders at the same time. This is commonly known as a wasted spark system. The two spark plugs are arranged so that one is fired on the power stroke of the engine and the other on the exhaust stroke of the opposing cylinder, offset by 360 degrees. After a complete rotation of the engine the two cylinders are now on the opposing strokes and the two spark plugs fire again but with opposite roles as before.

On a 4-cylinder engine, there are 2 coils with individual drivers that tend to operate cylinders 1 and 4, and 2 and 3. This means there is a dual spark every 180 degrees with one of those sparks wasted on an exhaust stroke of the opposing cylinder which is firing on the power stroke.

The only real difference between COP and other ignition systems is that each COP coil is mounted directly onto the spark plug, so the voltage goes directly to the plug electrodes without having to pass through a distributor or plug leads. This direct connection method delivers the strongest spark possible and improves the durability of the ignition system.

AT191-3

Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.